1. Field of the Invention
The present invention relates generally to a gas generating device and more specifically to a gas generating device which can selectively release gas at different rates and thus enable an air bag or the like type of inflatable safety restraining device, to be deployed at different speeds in accordance with different vehicle velocities.
2. Description of the Related Art
Recently, there has been a demand for controlling the rate of inflation of safety devices such as air-bag restraint systems, in accordance with variables such as passenger size and the velocity of the vehicle at time of collision. In the case of a low speed accident, the passenger or passengers will tend to be displaced within the vehicle (e.g. thrown forward) with far less force than in the case wherein a collision occurs at a relatively high speed. Accordingly, since an air bag requires a given amount of gas to achieve full deployment (inflation) and proper pressurization, if a passenger is being displaced within a vehicle under the influence of relatively small forces, then there is more time available to fully deploy the restraining air bag than in the case of a high speed crash. Therefore, the rate of inflation can be lower than that at which is necessary under high speed conditions.
In order to achieve the two speed inflation it has been proposed in U.S. Pat. No. 3,773,353 to Trowbridge et al. to provide two separate charges and to ignite one in the event that a slow inflation is required and to ignite both in the event of a high speed collision, thus achieving the very rapid inflation and deployment of the air bag which is necessary under such circumstances. In this device the charges are arranged within a housing which is filled with a non-toxic gas under pressure. This housing is sealed by a burst plate that is punched out by a piston and rod type of arrangement when a first of the two charges is detonated. This arrangement however, suffers from the drawback of being relatively complex and therefore, relatively expensive. For example, no less than three burst plate arrangements and two charges are necessary along with a housing arrangement which allows the two different charges to be separately stored and separately exposed to ignition devices such as squibs.
U.S. Pat. No. 3,905,515 to Allemann discloses another two-stage inflator assembly which utilizes two separate charges and which disposes the charge in a chamber which is used to store a non-noxious gas under pressure. However, this arrangement is even more complex than that in U.S. Pat. No. 3,773,353. In this arrangement a portion of the burst disc forms the head of a slidable shuttle valve member which is projectable into an exhaust passage to partially throttle the outflow of gases following a detonation of one or both of the two charges.